Turboprops
Setting up the Engine Specs and the Panel for turboprop aircraft is a typical design task that tends to confuse plane builders. To determine the proper engine specs, the first thing to do is to find out whether the engine in question features a "free" or a "fixed" turbine. * In a "fixed" turbine the drive shaft is directly driven (via a reduction gear) by the turbine section of the engine. * In a "free" turbine engine, there are actually two turbines, the first powering the compressor and the second, independend one powering the drive shaft. See also: Real-World Turboprops Which turbine type to use? Q: In cases where the designer is not reproducing a real-world aircraft, might there be some guidelines as to which type of engine might be better suited for certain applications? A: If counter-rotating constant-speed propellers are to be used, a fixed turbine should be chosen to ensure that the constant-speed gouvernor will work. For a free turbine, RPM of both propellers will fluctuate all the time, giving poor propulsive efficiency. Engine Specs Settings For a manually regulated turbine engine, the critical altitude should be set to 0 ft. If the aircraft is equipped with a FADEC (fully automated digital engine control), you have to try to find out the critical altitude of the engine type to set the correct critical altitude to get correct performance and control response. Turboprop-powered aircraft often have an emergency power setting that exceeds the standard power setting considerably. This can be implemented by setting a higher value for "max forward throttle" in the engine specs. (For example, the XV-15's Lycoming LTC1K-4K turboshafts are rated at 1250 shp continuous, and 1802 shp emergency power. So 1802/1250 = 1.44, so max forward throttle should be set at 1.44) Another way to simulate emergency power settings would be to use the green arcs and redlines on the instruments described below to indicate the normal operating range and the absolute maximum. (ToDo: Are both methods equivalent, or does one or the other - or both! - have disadvantages?) Note: If the autopilot's FLCH (flight level change) mode is used, it will go to 100% throttle. If this is set to simulate emergency power, that will make the autopilot climb at an unrealistically high power setting. Required Instruments For a "fixed" turbine: * One of engine rpm, N1, or propeller rpm. (Engine rpm was used mostly on older types. All give essentially the same value, as they are all connected physically, however propellor RPM is obviously measured after any clutch and transmission.) * One of power or torque. (Power was used mostly on older types). * ITT (Inter Turbine Temperature). For a "free" turbine: * One of engine rpm or N1. (Engine rpm was used mostly on older types). * One of propeller rpm or Np. (N2 is used for free turbine turbojet engines). * One of power or torque. (Power was used mostly on older types). * One for propeller RPM. * ITT (Inter Turbine Temperature). Each of these figures has its own redline value, and during turboprop operation, all of the limitations have to be observed. DEFINITIONS: * TORQUE - Measurement of torque given to the propeller from the power turbine measured in ft*lb(f) or percent of a nominal value. Excessive torque can destroy the drive train. * ITT - Inter Turbine Temperature measured with a temperature probe at some point within the engine as determined by the manufacturer. Excessive temperature can caues the turbine to fail. * N1 - Revolutions Per Minute (RPM) of the compressor section of a turbine engine (sometimes expressed percent of a nominal value). * N2 - RPM of the power turbine section of turbine engine. Rarely used in turboprop engines, used in high bypass turbojet engines (sometimes expressed percent of a nominal value). * RPM - RPM of the propeller which is most likely much lower than N2 RPM due to a speed reduction gearbox. Additional Instruments Typically, turboprop aircraft are also equipped with the following engine gauges: * Fuel flow indicator * Oil temperature gauge * Oil pressure gauge Optional instruments for an engine that turns a propeller through a speed reduction gearbox: * Transmission oil temperature. * Transmission oil pressure. External Links X-Plane.org forum discussion of turboprops